[davidmurdoch]

Seems very plausible. I haven't noticed a speed difference in my own pool with temperature changes, but that might just be because the temperature itself can be so distracting. But I do feel like a salty pool makes me faster (maybe from buoyancy), and borates makes the water feel smoother and less "sticky" (something about surface tension changes with borates, it even makes the surface look different).

[HPsquared]

Oh and of course the cooling effect. That's a factor for long distances especially if the water is too warm.